Solid-state imaging device and method for fabricating same

ABSTRACT

A solid-state imaging device includes a plurality of pixels two-dimensionally arrayed in a well region disposed on a semiconductor substrate, each pixel including a photoelectric conversion section having a charge accumulation region which accumulates signal charge; an element isolation layer which is disposed on the surface of the well region along the peripheries of the individual charge accumulation regions and which electrically isolates the individual pixels from each other; and a diffusion layer which is disposed beneath the element isolation layer and which electrically isolates the individual pixels from each other, the diffusion layer having a smaller width than that of the element isolation layer. Each charge accumulation region is disposed so as to extend below the element isolation layer and be in contact with or in close proximity to the diffusion layer.

CROSS REFERENCES TO RELATED APPLICATIONS

The subject matter of application Ser. No. 11/318,176, is incorporatedherein by reference. The present application is a divisional of U.S.Ser. No. 11/318,176, filed Dec. 23, 2005, which claims priority toJapanese Patent Application JP 2005-000727 filed in the Japanese PatentOffice on Jan. 5, 2005, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state imaging device in whichphotoelectric conversion efficiency per unit pixel is improved and to amethod for fabricating the solid-state imaging device.

2. Description of the Related Art

In solid-state imaging devices, photoelectric conversion efficiency perunit pixel has been improved by increasing the aperture ratio in theunit pixel or by increasing the light collection efficiency ofmicrolenses.

Recently, in solid-state imaging devices, a further increase in thenumber of pixels has been taking place, resulting in a decrease in thearea per unit pixel, and thus a further improvement in photoelectricconversion efficiency has been desired. Therefore, for example, in thecase of a photodiode, in a PN junction structure, the impurityconcentrations in the individual regions are increased, and thereby thephotoelectric conversion efficiency is improved (refer to KazuyaYonemoto, “CCD/CMOS imeji sensa no kiso to oyo (Basics and Applicationof CCD/CMOS Image Sensors)”, CQ Publishing Co., Ltd., pp. 92-94).

SUMMARY OF THE INVENTION

However, in the known solid-state imaging devices and fabricationmethods therefor, if the impurity concentrations of impurity regions ofphotoelectric converting elements are increased excessively, defectivepixels, such as white spots, occur frequently, resulting in problems.

It is desirable to provide a solid-state imaging device in whichphotoelectric conversion efficiency per unit pixel can be improved and amethod for fabricating such a solid-state imaging device.

According to an embodiment of the present invention, a solid-stateimaging device includes a plurality of pixels two-dimensionally arrayedin a well region disposed on a semiconductor substrate, each pixelincluding a photoelectric conversion section having a chargeaccumulation region which accumulates signal charge; an elementisolation layer which is disposed on the surface of the well regionalong the peripheries of the individual charge accumulation regions andwhich electrically isolates the individual pixels from each other; and adiffusion layer which is disposed beneath the element isolation layerand which electrically isolates the individual pixels from each other,the diffusion layer having a smaller width than that of the elementisolation layer, wherein each charge accumulation region is disposed soas to extend below the element isolation layer and be in contact with orin close proximity to the diffusion layer.

According to another embodiment of the present invention, in a methodfor fabricating a solid-state imaging device including a plurality ofpixels two-dimensionally arrayed in a well region disposed on asemiconductor substrate, each pixel including a photoelectric conversionsection having a charge accumulation region which accumulates signalcharge, the method includes the steps of forming an element isolationlayer on the surface of the well region, the element isolation layerelectrically isolating the individual pixels from each other; forming adiffusion layer so as to surround the individual charge accumulationregions and electrically isolate the individual pixels from each otherbeneath the element isolation layer; and forming the photoelectricconversion section for each pixel in the well region with thephotoelectric conversion sections being electrically isolated from eachother by the element isolation layer and the diffusion layer. Thephotoelectric conversion section formation step includes the substeps ofimplanting ions of an impurity for forming each charge accumulationregion in the well region; and thermally diffusing the impurityimplanted in the well region by the ion implantation substep to formeach charge accumulation region so that the charge accumulation regionextends below the element isolation layer and is brought in contact withor in close proximity to the diffusion layer.

According to another embodiment of the present invention, in a methodfor fabricating a solid-state imaging device including a plurality ofpixels two-dimensionally arrayed in a well region disposed on asemiconductor substrate, each pixel including a photoelectric conversionsection having a charge accumulation region which accumulates signalcharge, the method includes the steps of forming an element isolationlayer on the surface of the well region, the element isolation layerelectrically isolating the individual pixels from each other; forming adiffusion layer so as to surround the individual charge accumulationregions and electrically isolate the individual pixels from each otherbeneath the element isolation layer; and forming the photoelectricconversion section for each pixel in the well region with thephotoelectric conversion sections being electrically isolated from eachother by the element isolation layer and the diffusion layer. Thephotoelectric conversion section formation step includes the substeps ofimplanting first ions of an impurity for forming the charge accumulationregion of each photoelectric conversion section in the well region;masking above the charge accumulation region after the first ionimplantation substep; and implanting second ions of an impurity that isdifferent from the impurity used in the first ion implantation substepthrough the element isolation layer in the periphery of each chargeaccumulation region after the masking substep to form a chargeaccumulation extension so as to be brought in contact with or in closeproximity to the diffusion layer.

According to another embodiment of the present invention, in a methodfor fabricating a solid-state imaging device including a plurality ofpixels two-dimensionally arrayed in a well region disposed on asemiconductor substrate, each pixel including a photoelectric conversionsection having a charge accumulation region which accumulates signalcharge, the method includes the steps of forming a diffusion layer so asto surround the individual charge accumulation regions and electricallyisolate the individual pixels from each other; forming the photoelectricconversion section for each pixel in the well region with thephotoelectric conversion sections being electrically isolated from eachother by the diffusion layer, the photoelectric conversion sectionformation step including the substep of implanting ions of an impurityfor forming each charge accumulation region in the well region so thatthe charge accumulation region is brought in contact with or in closeproximity to the diffusion layer; and forming an element isolation layeron the surface of the well region after the ion implantation substep,the element isolation layer electrically isolating the individual pixelsfrom each other with the diffusion layer being located therebeneath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a first embodiment of the present invention;

FIG. 2 is a schematic sectional view illustrating a substantial part ofthe solid-state imaging device and a fabrication process step thereforaccording to the first embodiment of the present invention;

FIG. 3 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a second embodiment of the present invention;

FIG. 4 is a schematic sectional view illustrating a substantial part ofthe solid-state imaging device and a fabrication process step thereforaccording to the second embodiment of the present invention;

FIG. 5 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a third embodiment of the present invention; and

FIG. 6 is a schematic sectional view illustrating a substantial part ofthe solid-state imaging device and a fabrication process step thereforaccording to the third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solid-state imaging device according to any of the embodiments ofthe present invention is characterized in that the charge accumulationregion of each photoelectric conversion section is formed so as toextend below an element isolation layer and to be brought into contactwith or in close proximity to a diffusion layer. Preferably, after thecharge accumulation region of the photoelectric conversion section isformed by ion implantation, the charge accumulation region is allowed toextend below the element isolation layer by thermal diffusion.Alternatively, after the charge accumulation region of the photoelectricconversion section is formed by a first ion implantation process, thesurface of the well region corresponding to the charge accumulationregion surrounded with the element isolation layer is masked, and thenions of an impurity that is different from the impurity used in thefirst ion implantation process are implanted in the periphery of thecharge accumulation region to form a charge accumulation extensionregion. Alternatively, before the formation of the element isolationlayer, the charge accumulation region of the photoelectric conversionsection is formed by ion implantation.

In any of such structures, it is possible to increase the chargeaccumulation region (area) of the photoelectric conversion section ineach pixel, and the photoelectric conversion efficiency per unit pixelcan be improved.

First Embodiment

A solid-state imaging device and a method for fabricating the sameaccording to a first embodiment will be described below with referenceto the drawings.

FIG. 1 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a first embodiment of the present invention, and FIG. 2 isalso a schematic sectional view illustrating a substantial part of thesolid-state imaging device and a fabrication process step thereforaccording to the first embodiment of the present invention.

A solid-state imaging device 10 according to a first embodiment of thepresent invention includes a semiconductor substrate of a firstconductivity type, for example, an n-type silicon substrate 11, and asemiconductor well region of a second conductivity type, for example, ap-type semiconductor well region 12 is disposed on the substrate 11. Aplurality of pixels 20 are two-dimensionally arrayed in the p-typesemiconductor well region 12, each pixel 20 including a photodiode PDserving as a photoelectric conversion section and a transistor (notshown) for extracting signal charge accumulated in the photodiode PD. Anelement isolation layer 13 is disposed on the surface of the p-typesemiconductor well region 12, the element isolation layer 13electrically isolating the individual pixels 20 from each other. Ap-type diffusion layer 14 is disposed in the well region 12 beneath theelement isolation layer 13 so as to surround the individual chargeaccumulation regions, the diffusion layer 14 electrically isolating theindividual pixels 20 from each other. Reference numeral 15 represents aninsulating film disposed on the surface of the p-type semiconductor wellregion 12.

Each photodiode PD includes the p-type semiconductor well region 12 anda first conductivity type, i.e., n-type, charge accumulation region 17which accumulates signal charge. In this embodiment, the photodiode PDfurther includes a second conductivity type p⁺ accumulation layer 16disposed on the surface of the n-type charge accumulation region 17.Such a photodiode PD constitutes a sensor having a hole accumulationdiode (HAD) structure.

In the photodiode PD, the p⁺ accumulation layer 16 suppresses darkcurrent which is caused by an interface state and results in whitespots. The p-type diffusion layer 14 also has a function of isolatingthe photodiodes PD from each other in the depth direction.

A method for fabricating the solid-state imaging device 10 will now bedescribed with reference to FIGS. 1 and 2.

First, as shown in FIG. 1, a p-type semiconductor well region 12 isformed on an n-type silicon substrate 11. Subsequently, usingphotolithography, an isolation pattern is formed by patterning on thep-type semiconductor well region 12, the isolation pattern isolating theindividual pixels 20 each including a photodiode PD serving as aphotoelectric conversion section, and then a p-type diffusion layer 14is formed by performing ion implantation once or a plurality of times inthe depth direction. In such a case, the dosage during the ionimplantation is about 1×10¹² cm⁻², and the width d1 of the p-typediffusion layer 14 is about 0.05 to 10 μm. By setting the width d1 ofthe p-type diffusion layer 14 in the range described above and as smallas allowed by the processing capability so that the width d1 is smallerthan the line width d2 of the element isolation layer 13, an extensionregion 18 is produced in the p-type semiconductor well region 12 belowthe element isolation layer 13 in the place surrounded with the p-typediffusion layer 14 in each pixel, the extension region 18 substantiallyenlarging the charge accumulation area of the n-type charge accumulationregion 17.

Subsequently, using photolithography, an element isolation pattern isformed by patterning on the surface of the p-type semiconductor wellregion 12, the element isolation pattern isolating the individual pixels20 each including the photodiode PD serving as a photoelectricconversion section, and thus an element isolation layer 13, for example,composed of SiO₂ is formed. Then, ions are implanted through eachopening 13A of the element isolation layer 13 into the p-typesemiconductor well region 12, for example, at a dosage of about 1×10¹²cm⁻². Thereby, an n-type charge accumulation region 17 having an areacorresponding to the opening 13A is formed. Subsequently, p-type ionswith a high concentration, for example, of 5×10¹⁷ cm⁻³ or more areimplanted in the surface of the n-type charge accumulation region 17,followed by diffusion, and thereby a p⁺ accumulation layer 16 is formed.

Subsequently, a solid-state imaging device 10 having a structure shownin FIG. 1 is placed in a thermal diffusion furnace (not shown) andheated at a predetermined temperature, for example, in an atmosphere of900° C., for a predetermined time, for example, about 10 minutes, tothermally diffuse the impurity of the n-type charge accumulation region17 in the p-type semiconductor well region 12. Thereby, as shown in FIG.2, the n-type charge accumulation region 17 is enlarged in the depthdirection of the photodiode PD and in a direction perpendicular to thedepth direction, at least in the direction perpendicular to the depthdirection of the photodiode PD so that the peripheral part of the n-typecharge accumulation region 17 is in contact with or in close proximityto the p-type diffusion layer 14 below the element isolation layer 13.Here, the term “being in close proximity” means that the n-type chargeaccumulation region 17 extends toward the p-type diffusion layer 14 byat least half of the width of the extension region 18.

In the solid-state imaging device 10 and the method for fabricating thesame according to the first embodiment, in the step of forming thephotodiode PD serving as the photoelectric conversion section, after thecharge accumulation region 17 of the photodiode PD is formed by ionimplantation, the charge accumulation region 17 is allowed to extendbelow the element isolation layer 13 and brought in contact with or inclose proximity to the diffusion layer 14. Therefore, the chargeaccumulation region 17 can be easily formed also below the elementisolation layer 13. Consequently, it is possible to increase the chargeaccumulation region (area) of the photoelectric conversion section ineach pixel, and the photoelectric conversion efficiency per unit pixelcan be improved. Furthermore, it is not necessary to increase theimpurity concentrations of impurity regions of photoelectric convertingelements as has been necessary in the past, and thus it is possible toreduce occurrence of defective pixels, such as white spots.

Second Embodiment

A solid-state imaging device and a method for fabricating the sameaccording to a second embodiment of the present invention will bedescribed below with reference to FIGS. 3 and 4.

FIG. 3 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a second embodiment of the present invention, and FIG. 4 isalso a schematic sectional view illustrating a substantial part of thesolid-state imaging device and a fabrication process step thereforaccording to the second embodiment of the present invention.

As in the first embodiment, a solid-state imaging device 30 according tothe second embodiment includes a semiconductor substrate of a firstconductivity type, for example, an n-type silicon substrate 11, and asemiconductor well region of a second conductivity type, for example, ap-type semiconductor well region 12 is disposed on the substrate 11. Aplurality of pixels 20 are two-dimensionally arrayed in the p-typesemiconductor well region 12, each pixel 20 including a photodiode PDserving as a photoelectric conversion section. An element isolationlayer 13 is disposed on the surface of the p-type semiconductor wellregion 12, the element isolation layer 13 electrically isolating theindividual pixels 20 from each other. A p-type diffusion layer 14 isdisposed in the well region 12 beneath the element isolation layer 13 soas to surround the individual charge accumulation regions, the diffusionlayer 14 electrically isolating the individual pixels 20 from eachother. Reference numeral 15 represents an insulating film disposed onthe surface of the p-type semiconductor well region 12.

Each photodiode PD includes the p-type semiconductor well region 12 anda first conductivity type, i.e., n-type, charge accumulation region 17which accumulates signal charge. In this embodiment, the photodiode PDfurther includes a second conductivity type p⁺ accumulation layer 16disposed on the surface of the n-type charge accumulation region 17.Such a photodiode PD constitutes a sensor having a HAD structure.

In the photodiode PD, the p⁺ accumulation layer 16 suppresses darkcurrent which is caused by an interface state and results in whitespots. The p-type diffusion layer 14 also has a function of isolatingthe photodiodes PD from each other in the depth direction.

A method for fabricating the solid-state imaging device 30 will now bedescribed with reference to FIGS. 3 and 4.

First, referring to FIG. 3, as in the first embodiment, a p-typesemiconductor well region 12 is formed on an n-type silicon substrate11. Subsequently, using photolithography, an isolation pattern is formedby patterning on the p-type semiconductor well region 12, the isolationpattern isolating the individual pixels 20 each including a photodiodePD serving as a photoelectric conversion section, and then a p-typediffusion layer 14 is formed by performing ion implantation once or aplurality of times in the depth direction. In such a case, the dosageduring the ion implantation is about 1×10¹² cm⁻², and the width d1 ofthe p-type diffusion layer 14 is about 0.05 to 10 μm. By setting thewidth d1 of the p-type diffusion layer 14 in the range described aboveand as small as allowed by the processing capability so that the widthd1 is smaller than the line width d2 of the element isolation layer 13,an extension region 18 is produced in the p-type semiconductor wellregion 12 below the element isolation layer 13 in the place surroundedwith the p-type diffusion layer 14 in each pixel, the extension region18 substantially enlarging the charge accumulation area of the n-typecharge accumulation region 17.

Subsequently, as in the first embodiment, using photolithography, anelement isolation pattern is formed by patterning on the surface of thep-type semiconductor well region 12, the element isolation patternisolating the individual pixels 20 each including the photodiode PDserving as a photoelectric conversion section, and thus an elementisolation layer 13, for example, composed of SiO₂ is formed. Then, ionsare implanted through each opening 13A of the element isolation layer 13into the p-type semiconductor well region 12, for example, at a dosageof about 1×10¹² cm⁻². Thereby, an n-type charge accumulation region 17having an area corresponding to the opening 13A is formed. Subsequently,p-type ions with a high concentration, for example, of 5×10¹⁷ cm⁻³ ormore are implanted in the surface of the n-type charge accumulationregion 17, followed by diffusion, and thereby a p⁺ accumulation layer 16is formed.

Subsequently, as shown in FIG. 4, after the ion implantation step forforming the n-type charge accumulation region 17 is completed, thesurface region of the photodiode PD corresponding to an opening 13Asurrounded by the element isolation layer 13 is masked with a resistfilm 21. Then, ions of an impurity that is different from the impurityused in the ion implantation for the n-type charge accumulation region17 are implanted in an extension region 18 between the outer peripheryof the n-type charge accumulation region 17 and the inner periphery ofthe p-type diffusion layer 14 through the element isolation layer 13.Thereby, a charge accumulation extension 19 is formed so as to bebrought in contact with or in close proximity to the inner periphery ofthe p-type diffusion layer 14, the charge accumulation extension 19substantially enlarging the charge accumulation area of the n-typecharge accumulation region 17. Here, the term “being in close proximity”means that the charge accumulation extension 19 extends toward thep-type diffusion layer 14 by at least half of the width of the extensionregion 18.

In the solid-state imaging device 30 and the method for fabricating thesame according to the second embodiment, in the step of forming thephotodiode PD serving as the photoelectric conversion section, after thecharge accumulation region 17 is formed by ion implantation, the surfaceregion of the photodiode PD corresponding to the opening 13A surroundedby the element isolation layer 13 is masked with the resist film 21, andthen ions of an impurity that is different from the impurity used forthe n-type charge accumulation region 17 are implanted in the extensionregion 18 facing the outer periphery of the n-type charge accumulationregion 17 to form the charge accumulation extension 19 so as to bebrought in contact with or in close proximity to the diffusion layer 14.Therefore, the charge accumulation region 17 can be easily formed alsobelow the element isolation layer 13. Consequently, it is possible toincrease the charge accumulation region (area) of the photoelectricconversion section in each pixel, and the photoelectric conversionefficiency per unit pixel can be improved. Furthermore, it is notnecessary to increase the impurity concentrations of impurity regions ofphotoelectric converting elements as has been necessary in the past, andthus it is possible to reduce occurrence of defective pixels, such aswhite spots.

Third Embodiment

A solid-state imaging device and a method for fabricating the sameaccording to a third embodiment of the present invention will bedescribed below with reference to FIGS. 5 and 6.

FIG. 5 is a schematic sectional view illustrating a substantial part ofa solid-state imaging device and a fabrication process step thereforaccording to a third embodiment of the present invention, and FIG. 6 isalso a schematic sectional view illustrating a substantial part of thesolid-state imaging device and a fabrication process step thereforaccording to the third embodiment of the present invention.

As in the first embodiment, a solid-state imaging device 40 according tothe third embodiment includes a semiconductor substrate of a firstconductivity type, for example, an n-type silicon substrate 11, and asemiconductor well region of a second conductivity type, for example, ap-type semiconductor well region 12 is disposed on the substrate 11. Aplurality of pixels 20 are two-dimensionally arrayed in the p-typesemiconductor well region 12, each pixel 20 including a photodiode PDserving as a photoelectric conversion section. An element isolationlayer 13 is disposed on the surface of the p-type semiconductor wellregion 12, the element isolation layer 13 electrically isolating theindividual pixels 20 from each other. A p-type diffusion layer 14 isdisposed in the well region 12 beneath the element isolation layer 13 soas to surround the individual charge accumulation regions, the diffusionlayer 14 electrically isolating the individual pixels 20 from eachother. Reference numeral 15 represents an insulating film disposed onthe surface of the p-type semiconductor well region 12.

Each photodiode PD includes the p-type semiconductor well region 12 anda first conductivity type, i.e., n-type, charge accumulation region 17which accumulates signal charge. In this embodiment, the photodiode PDfurther includes a second conductivity type p⁺ accumulation layer 16disposed on the surface of the n-type charge accumulation region 17.Such a photodiode PD constitutes a sensor having a HAD structure.

In the photodiode PD, the p⁺ accumulation layer 16 suppresses darkcurrent which is caused by an interface state and results in whitespots. The p-type diffusion layer 14 also has a function of isolatingthe photodiodes PD from each other in the depth direction.

A method for fabricating the solid-state imaging device 40 will now bedescribed with reference to FIGS. 5 and 6.

First, referring to FIG. 5, as in the first embodiment, a p-typesemiconductor well region 12 is formed on an n-type silicon substrate11. Subsequently, using photolithography, an isolation pattern is formedby patterning on the p-type semiconductor well region 12, the isolationpattern isolating the individual pixels 20 each including a photodiodePD serving as a photoelectric conversion section, and then a p-typediffusion layer 14 is formed by performing ion implantation once or aplurality of times in the depth direction. In such a case, the dosageduring the ion implantation is about 1×10¹² cm⁻², and the width d1 ofthe p-type diffusion layer 14 is about 0.05 to 10 μm. By setting thewidth d1 of the p-type diffusion layer 14 in the range described aboveand as small as allowed by the processing capability so that the widthd1 is smaller than the line width d2 of the element isolation layer 13,an extension region is produced in the p-type semiconductor well region12 below the element isolation layer 13 in the place surrounded by thep-type diffusion layer 14 in each pixel, the extension regionsubstantially enlarging the charge accumulation area of the n-typecharge accumulation region 17.

Subsequently, ions are implanted at a dosage of about 1×10¹² cm⁻² in thesurface of the p-type semiconductor well region 12 for each pixel beforethe formation of the element isolation layer 13, and thereby an n-typecharge accumulation region 17 having dimensions capable of being incontact with or in close proximity to the inner periphery of the p-typediffusion layer 14 is formed. Subsequently, p-type ions with a highconcentration, for example, of 5×10¹⁷ cm⁻³ or more are implanted in thesurface of the n-type charge accumulation region 17, followed bydiffusion, and thereby a p⁺ accumulation layer 16 is formed.

Subsequently, using photolithography, an element isolation pattern isformed by patterning on the surface of the p-type semiconductor wellregion 12, the element isolation pattern isolating the individual pixels20 each including the photodiode PD serving as a photoelectricconversion section, and thus an element isolation layer 13, for example,composed of SiO₂ is formed on the p-type semiconductor well region 12.

In the solid-state imaging device 40 and the method for fabricating thesame according to the third embodiment, in the step of forming thephotodiode PD serving as the photoelectric conversion section, prior tothe formation of the element isolation layer 13, the n-type chargeaccumulation region 17 having dimensions capable of being in contactwith or in close proximity to the inner periphery of the p-typediffusion layer 14 is formed. After the formation of the chargeaccumulation region 17, the element isolation layer 13 is formed.Therefore, the charge accumulation region 17 can be easily formed alsobelow the element isolation layer 13. Consequently, it is possible toincrease the charge accumulation region (area) of the photoelectricconversion section in each pixel, and the photoelectric conversionefficiency per unit pixel can be improved. Furthermore, it is notnecessary to increase the impurity concentrations of impurity regions ofphotoelectric converting elements as has been necessary in the past, andthus it is possible to reduce occurrence of defective pixels, such aswhite spots.

In each of the first to third embodiments described above, thephotodiode PD is used as the sensor having a hole accumulation diode(HAD) structure in which the p⁺ accumulation layer 16 is disposed on then-type charge accumulation region 17. However, the present invention isnot limited thereto. The photodiode PD may have a structure without thep⁺ accumulation layer 16.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

In the solid-state imaging device according to the embodiment of thepresent invention, since the charge accumulation region of eachphotoelectric conversion section is disposed so as to extend below theelement isolation layer and be in contact with or in close proximity tothe diffusion layer, it is possible to increase the charge accumulationregion (area) of the photoelectric conversion section in each pixel, andthe photoelectric conversion efficiency per unit pixel can be improved.Furthermore, it is not necessary to increase the impurity concentrationsof impurity regions of photoelectric converting elements as has beennecessary in the past, and thus it is possible to reduce occurrence ofdefective pixels, such as white spots.

In the method for fabricating a solid-state imaging device according toone of the embodiments of the present invention, in the photoelectricconversion section formation step, after the charge accumulation regionof each photoelectric conversion section is formed by ion implantation,the charge accumulation region is allowed to extend below the elementisolation layer and brought in contact with or in close proximity to thediffusion layer by thermal diffusion. Therefore, the charge accumulationregion can be easily formed also below the element isolation layer.Consequently, it is possible to increase the charge accumulation region(area) of the photoelectric conversion section in each pixel, and thephotoelectric conversion efficiency per unit pixel can be improved.Furthermore, it is not necessary to increase the impurity concentrationsof impurity regions of photoelectric converting elements as has beennecessary in the past, and thus it is possible to reduce occurrence ofdefective pixels, such as white spots.

In the method for fabricating a solid-state imaging device according toone of the embodiments of the present invention, in the photoelectricconversion section formation step, after the charge accumulation regionof each photoelectric conversion section is formed by the first ionimplantation substep, the surface region of the semiconductor substratecorresponding to each charge accumulation region surrounded with theelement isolation layer is masked, and in such a state, ions of animpurity that is different from the impurity used in the first ionimplantation substep are implanted in the periphery of the chargeaccumulation region to form a charge accumulation extension so as to bebrought in contact with or in close proximity to the diffusion layer.Therefore, the charge accumulation region can be easily formed alsobelow the element isolation layer. Consequently, it is possible toincrease the charge accumulation region (area) of the photoelectricconversion section in each pixel, and the photoelectric conversionefficiency per unit pixel can be improved. Furthermore, it is notnecessary to increase the impurity concentrations of impurity regions ofphotoelectric converting elements as has been necessary in the past, andthus it is possible to reduce occurrence of defective pixels, such aswhite spots.

In the method for fabricating a solid-state imaging device according toone of the embodiments of the present invention, in the photoelectricconversion section formation step, before the element isolation layerformation step, the charge accumulation region of each photoelectricconversion section is formed by ion implantation, and after theformation of the charge accumulation region, the element isolation layeris formed. Therefore, the charge accumulation region can be easilyformed also below the element isolation layer. Consequently, it ispossible to increase the charge accumulation region (area) of thephotoelectric conversion section in each pixel, and the photoelectricconversion efficiency per unit pixel can be improved. Furthermore, it isnot necessary to increase the impurity concentrations of impurityregions of photoelectric converting elements as has been necessary inthe past, and thus it is possible to reduce occurrence of defectivepixels, such as white spots.

1.-2. (canceled)
 3. A method for fabricating a solid-state imagingdevice including a plurality of pixels two-dimensionally arrayed in awell region disposed on a semiconductor substrate, each pixel includinga photoelectric conversion section having a charge accumulation regionwhich accumulates signal charge, the method comprising the steps of:forming an element isolation layer on the surface of the well region,the element isolation layer electrically isolating the individual pixelsfrom each other; forming a diffusion layer so as to surround theindividual charge accumulation regions and electrically isolate theindividual pixels from each other beneath the element isolation layer;and forming the photoelectric conversion section for each pixel in thewell region with the photoelectric conversion sections beingelectrically isolated from each other by the element isolation layer andthe diffusion layer, wherein the photoelectric conversion sectionformation step includes the substeps of: implanting ions of an impurityfor forming each charge accumulation region in the well region; andthermally diffusing the impurity implanted in the well region by the ionimplantation substep to form each charge accumulation region so that thecharge accumulation region extends below the element isolation layer andis brought in contact with or in close proximity to the diffusion layer.4. A method for fabricating a solid-state imaging device including aplurality of pixels two-dimensionally arrayed in a well region disposedon a semiconductor substrate, each pixel including a photoelectricconversion section having a charge accumulation region which accumulatessignal charge, the method comprising the steps of: forming an elementisolation layer on the surface of the well region, the element isolationlayer electrically isolating the individual pixels from each other;forming a diffusion layer so as to surround the individual chargeaccumulation regions and electrically isolate the individual pixels fromeach other beneath the element isolation layer; and forming thephotoelectric conversion section for each pixel in the well region withthe photoelectric conversion sections being electrically isolated fromeach other by the element isolation layer and the diffusion layer,wherein the photoelectric conversion section formation step includes thesubsteps of: implanting first ions of an impurity for forming the chargeaccumulation region of each photoelectric conversion section in the wellregion; masking above the charge accumulation region after the first ionimplantation substep; and implanting second ions of an impurity that isdifferent from the impurity used in the first ion implantation substepthrough the element isolation layer in the periphery of each chargeaccumulation region after the masking substep to form a chargeaccumulation extension so as to be brought in contact with or in closeproximity to the diffusion layer.
 5. A method for fabricating asolid-state imaging device including a plurality of pixelstwo-dimensionally arrayed in a well region disposed on a semiconductorsubstrate, each pixel including a photoelectric conversion sectionhaving a charge accumulation region which accumulates signal charge, themethod comprising the steps of: forming a diffusion layer so as tosurround the individual charge accumulation regions and electricallyisolate the individual pixels from each other; forming the photoelectricconversion section for each pixel in the well region with thephotoelectric conversion sections being electrically isolated from eachother by the diffusion layer, the photoelectric conversion sectionformation step including the substep of implanting ions of an impurityfor forming each charge accumulation region in the well region so thatthe charge accumulation region is brought in contact with or in closeproximity to the diffusion layer; and forming an element isolation layeron the surface of the well region after the ion implantation substep,the element isolation layer electrically isolating the individual pixelsfrom each other with the diffusion layer being located therebeneath. 6.The method for fabricating the solid-state imaging device according toany one of claims 3, 4, and 5, wherein the photoelectric conversionsection includes a hole accumulation layer disposed on the chargeaccumulation region.